There have been known sewing machines of a type which includes a vertically driven needle bar, a sewing needle fixed to a lower end portion of the needle bar, a rotary member mounted concentrically with the needle bar and freely rotatable about the axis and a guide fixed to the rotary member for guiding the string material (i.e., string-shaped embroidering member, such as a tape or cord) to a sewing position of the sewing needle. The sewing machines of the type operate to sew the string material to a fabric through lock stitching, by the rotation of the rotary member being appropriately controlled in accordance with a moving direction of a fabric based on embroidery data and by the orientation of the guide being appropriately varied to optimize the direction in which the string material is guided to the sewing position of the sewing needle. One example of such sewing machines is known from Japanese Patent Application Laid-open Publication No. HEI-3-286797. The No. HEI-3-286797 publication discloses a sewing machine of the above-mentioned type, where a bobbin having a string material wound thereon has an increased size by being disposed in a space above the needle bar. The bobbin having the string material wound thereon is mounted on a bobbin shaft supported at its opposite ends by a pair of support members fixed to a machine frame, and the bobbin is supported at its opposite end portions by a pair of retaining members mounted on the bobbin shaft. First guide roller for winding the string material from the bobbin to invert upwardly the feed direction of the string material is rotatably provided beneath a substantial middle region of the bobbin. Roller support frame is rotatably mounted at its proximal end portion to the pair of support members, and a second guide roller for winding thereon the string material, inverted by the first guide roller, and further inverting downward the direction of the string material is rotatably mounted to a portion of the roller support frame corresponding in position to the substantial middle region of the bobbin. Coil spring is provided on the roller support frame for normally biasing the roller support frame in the upward direction, and a brake member is also provided on the roller support frame in such a manner that it can be brought to frictional contact with the retaining members when necessary.
The string material paid out from the bobbin is guided to the sewing position via the first guide roller and second guide roller. As the second guide roller is pulled by the string material in accordance with a progression of sewing of the string material, the roller support frame is caused to pivot downward and the bobbin is rotated by being pulled by the string material so that the string material is paid out from the bobbin. As the tension of the string material decreases by the paying-out of the string material, the roller support frame is caused to pivot upward by the biasing force of the coil spring. Once the roller support frame is caused to pivot further upward by the string material being paid out sufficiently, the brake member is brought to frictional contact with the retaining members, which terminates the rotation of the bobbin and thereby inhibits inertial rotation (overrun) of the bobbin. In such a sewing machine, where the bobbin having the string material wound thereon is located above the needle bar, the bobbin can have an increased size so that an increased amount of the string material can be wound on the bobbin.
In the conventionally-known sewing machines like the one disclosed in the HEI-3-286797 publication, as the string material is paid out from the bobbin and sewn onto a fabric, the bobbin is rotated by being pulled by the string material, in accordance with a progression of the sewing operation, so that the string material is further paid out from the bobbin, as set forth above. However, because the large-size bobbin having an increased amount of the string material wound thereon would have an increased overall weight, an extremely great force would be required to rotate the bobbin in order to pay out the string material. Thus, if the bobbin is very heavy in weight, the rotation of the bobbin tends to be slow so that the paying-out of the string material is delayed behind the sewing operation, and the bobbin tends to start rotating rapidly so that the string material may be paid out more than necessary. Further, depending on the weights of the bobbin and string material, the bobbin may fail to rotate even when it is pulled by the string material being sewn, in which case the string material may not be paid out at all. Thus, the convention sewing machines would suffer from the inconveniences that the string material can not be paid out smoothly, the string material can not be sewn accurately and beautifully or aesthetically and the sewing of the string material is undesirably halted.